Boiler pressure high when running, expansion tank and relief valve are fine

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Old 10-18-15, 07:07 PM
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Boiler pressure high when running, expansion tank and relief valve are fine

My system is single zone gas fired water boiler. Every time the boiler is running the pressure creeps up to 30 PSI blowing off the relief valve (at around 180F). From other posts I see most of the times the issue is with the expansion tank so I went ahead and replaced it but I was wrong. That didn't resolve the issue, the pressure still goes up when the boiler is running (hits 30 PSI around 180F).
The expansion tank is bladder type and I made sure the pressure was 12 PSI before mounting.

Today when the pressure was around 30 PSI I tried knocking on the ex tank and found that the water gets filled only half way, why is the other half staying empty (same thing happened with my old expansion tank)? isn't the bladder supposed to expand? I was curious and checked the pressure on the expansion tank and it was only 22PSI ! I am aware, to accurately check the pressure I am supposed to drain the system first but even with the water and air pressure combined it was only 22PSI while the relief valve was blowing at 30PSI (the tridicator was also showing 30PSI). What could be preventing the bladder to expand freely? the tank is not placed hanging but its mounted on its head. I am not sure if this may be a factor.

I have also tried to diagnose the relief valve and it seem to be fine. It maintains pressure at 12PSI all the time when off. The extra water lost due to relief valve blowing off is compensated as soon as the pressure drops below 12PSI.

Any thoughts? Thanks in advance.
 
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Old 10-18-15, 11:08 PM
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Today when the pressure was around 30 PSI I tried knocking on the ex tank and found that the water gets filled only half way, why is the other half staying empty
Think about it. If it was full of water there could be no air cushion and water can't be compressed.

You don't know how accurate the boiler gauge is. I'd recommend picking up one like in the picture below and connect to the boiler drain valve.

Rain Bird 3 in. P2A Water Pressure Gauge-P2A - The Home Depot
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Old 10-19-15, 06:11 AM
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Except that gauge is 200 PSI and won't read well at low pressures.

the tank is not placed hanging but its mounted on its head. I am not sure if this may be a factor.
It will still do it's job. It might not last as long but don't worry about that now.

You replaced it with the same size tank? and it is one for HEATING systems and NOT for potable water systems?

Read this:

http://www.doityourself.com/forum/bo...ure-gauge.html

And I think you have already read this:

http://www.doityourself.com/forum/bo...sion-tank.html

You can't really tell anything by tapping on the tank.

When you measure the tank with pressure on the water side, you should see the HIGHER of the two pressures, either the tank pre-charge if the boiler pressure is at or below the pre-charge, OR the boiler pressure, if it is above the pre-charge.

Tire gauge out of spec? Probably... most of them are junk.

Boiler gauge bad? Maybe... and just because it says 30 PSI when the relief valve opens, does not mean that the gauge is LINEAR. It could be correct at 30, but wrong at other pressures.

Combination of both gauges out of spec, possible.
 
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Old 10-19-15, 06:14 AM
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It maintains pressure at 12PSI all the time when off
With the valve ahead of the 'pressure reducing valve' OPEN?

It never creeps up above 12 PSI?

What kind of heat emitters are on the system? FIN-TUBE (copper tube, aluminum fins) baseboard? or CAST IRON radiators?

Verify the boiler gauge.

Verify your tire gauge.

Reset the air pressure in the expansion tank after verifying your tire gauge.
 
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Old 10-19-15, 08:05 AM
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NJT, yes the main feed valve ahead of pressure reducing valve is open. Correct, pressure never creeps up to over 12PSI when off. Rise in pressure happens when the boiler is on. I have cast iron radiators and Yes I replaced it with same size tank which is meant for non potable water system (for closed loop system). I would like to get a new tire gauge, which one would you recommend?

PJmax, thanks for recommending and sending the link to the gauge, I was trying to find this on Amazon yesterday but was confused which one to order. This one looks perfect, will pick up from HD in a day or two.

Some additional info about my boiler that may help diagnose the problem - The boiler was replaced/installed about 4 years ago, haven’t had this issue before. The radiators are cast iron and the pipes are large heating pipes (not copper, old school type, house is over 100 yrs old).

Here is a video I originally took for a different issue (that has been resolved since) but can be used here so you can get a better idea of the boiler set up.

https://www.youtube.com/watch?v=8SEk...ature=youtu.be
 
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Old 10-19-15, 08:20 AM
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I replaced it with same size tank
In the video it appears to be a size '30' and based on the fact that you have the large pipes and cast iron radiators, that tank is most likely undersized for your system.

Do you have a history with the system, or is this a 'new to you' home?

In other words, do you know if the system ever worked properly with that tank size?

Chances are good that someone replaced an older steel compression tank strapped into the joists with that new tank...
 
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Old 10-19-15, 02:31 PM
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that is correct NJT. Its size 30 tank. The boiler was installed 4 years ago along with the tank you see in the video. I haven't had any problems with the pressure in the past. Also, the old boiler, which I think was 35 yrs old when we moved into the house, had the same size tank. And as you said its likely that previous owners had the bigger tank replaced by a size "30" tank.
 
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